1813.] 



Scientific Intelligence, 



303 



of Iceland crystal, and is transmitted through the opposite sur- 

 face, it is separated into two pencils, one of which proceeds in 

 tlie direction of the incident ray, while the other forms with it 

 an angle of 6° 16'. The first of these pencils is said to expe- 

 rience the usual or ordinary refraction, and the other the 

 iimisual or extraordinary refraction. If the luminous object 

 from which the ray of light proceeds he looked at through the 

 crystal, two images of it will be distinctly seen, even when the 

 rhomboid is turned round the axis of vision. If another rhom- 

 boid of Iceland spar is placed behind the first, in a similar 

 position, the pencil refracted in the ordinary way by the first will 

 be so also by the second, and the same thing holds with the 

 extraordinarily refracted pencil — none of the pencils being 

 separated into two, as before. But if the second rhomboid is 

 turned slowly round, while the first remains stationary, each of 

 the pecils begin to separate into two; and when the eighth 

 part of a revolution is completed, the whole of each of the 

 pencils is divided into two portions. When the fourth part of a 

 revolution is completed, the pencil refracted in the ordinary way 

 by the first crystal will be refracted in the extraordinary way only 

 by the second, and the pencil refracted in the extraordinary way 

 1)y the first will be refracted in the ordinary way only by the 

 second; so that the four pencils will be again reduced to two. 

 At the end of f, -f-, and -J, of a revolution, the same phcnom.ena 

 will be exhibited as at the end of of a revolution. At the end 

 of \ and -I of a revolution, the same phenomena will be seen as 

 at the first position of the crystals, and at the end of f of a 

 revolution. 



If we look at a luminous object through the two rhomboids, 

 we shall at the commencement of the revolution see only two 

 images, viz. one of the least, and of the greatest refracted 

 images. At the end of \ of a revolution four images will be 

 seen, and so on as in the preceding example. 



It is obvious that the light which forms these images has 

 suflfered some new modification, or acquired some new property, 

 which prevented it in particular parts of a revolution from pene- 

 trating the second rhomboid. This property has been called 

 polarization ; and light is said to he polarized by passing through 

 a rhomboid of calcareous spar, or any other doubly refracting 

 crystal. 



Almost all crystallized substances possess the property of 

 double refraction, and consequently the power of polarizing 

 light. The most important of these, arranged in the order of 

 their refractive power, according to the experiments of Dr. 

 Brewster, are the following : — 



